I've been searching for pumps I can use for medium to large drain back solar systems on tall buildings. Typical systems are 15 to 20 GPM, 40 to 50 feet of lifting head at filling time. I have some Taco 2400-40 and 2400-50s that work well with fixed speed controls. However, I want to use the variable speed outputs on RESOL controls (relayed up to proper amperage of course). What brands/models are installers out there using? Unfortunately the system I am building now is too tall for the Grundfos UPS series, otherwise I'd go there... No, I do not want to double-pump because we use water (bad failure mode!).
The Grundfos VersaFlow series, looks like that has 40- 50' head, if you want to use Grundfos.
Caleffi offers a solid state relay that connects to the R-1 on the i-solar control. We have used this to control large pumps with variable speed function.. The relay we offer is rated 25 amp 24- 250VAC. We have several installed in the field on Magna pumps. Check with Taco to see if their pumps could power from a triac.
We also connected a 5 hp shop vac to the relay to test it in the shop. On large loads you need to use a heat sink on the back side of the relay for heat dissipation.
Grundfos does offer PWM, high efficiency solar pumps that plug directly into the i-solar controls, but they are Euro versions in 230V. I used this 25-105 one on my solar drainback, until my array out-grew the pumps capacity. It's a true PWM pump.